This invention relates to an improved point source X-ray focusing device. The device utilizes synthetic layered structures which are free from the constraints of crystalline symmetries. The layers are formed on a focusing element surface which is selected to collect, reflect and focus the maximum X-ray flux from the point source to a focus point for a particular wavelength of interest.
Commercial X-ray dispersive structures are formed from crystalline structures such as LiF, metal acid phthalates (map), pyrolytic graphite and Langmuir-Blodgett (LB) films. These materials have very restrictive lattice spacing constraints. In addition, the LB and map devices have severe environmental limitations and must be operated near room temperature in a dry environment. LB devices are not appropriate for very high vacuum applications since under certain conditions they can evolve contaminants. They are also inappropriate for high incident beam energy applications since they can decompose. They have poor mechanical integrity, such as scratch resistance, mechanical breaking strength and resistance to abrasion. Further, all of the prior structures have lower reflectivities than desired.
Numerous attempts to construct both natural and new crystalline analogue materials have been made with the aim of extending the X-ray properties heretofore limited by the availability of natural crystalline materials. One such attempt is compositional modulation by molecular beam epitaxy (MBE) deposition on single crystal substrates. For example, in Dingle et al., U.S. Pat. No. 4,261,771, the fabrication of monolayer semiconductors by one MBE technique is described. These modulated prior art structures are typically called "superlattices." Superlattices are developed on the concept of layers of materials forming homo or hetero epitaxially grown planes or film layers resulting in a one-dimensional periodic potential. Typically, the largest period in these superlattices is on the order of a few hundred Angstroms; however, monatomic layered structures have also been constructed.
The superlattices can be characterized by the format of a number of layer pairs formed by a layer of A (such as GaAs) followed by a layer of B (such as AlAs), etc.; formed on a single crystal substrate. The desired superlattice is a single crystal synthetic material with good crystalline quality and long range order. The thickness of each layer pair (A and B) is defined as the "d" spacing. These structures are not appropriate for most reflective or dispersive structures due to the small electron density contrast between the layers. These structures being essentially single crystals with extra super lattice periodicities also suffer from restrictive d spacing, associated with the constraint that the entire structure be a single crystal.
In addition to the MBE type of superlattice construction techniques, other researchers have developed layered synthetic microstructures (lsm) utilizing other forms of vapor deposition, including diode and magnetron sputtering, reactive gas injection and standard multisource evaporation. The layer dimensions are controlled by shutters or moving the substrates relative to the material sources or with combinations of shutters and relative motion. In the case of multisource evaporation, the required thickness control is achieved by monitoring the X-ray reflectivity of the film in situ as the deposition is being made. The materials reported have been formed from crystalline layers, noncrystalline layers and mixtures thereof; however, generally the efforts so far reported are directed at the synthesis of superlattice-type structures by precisely reproducing the deposition conditions on a periodic reoccurring basis. Some of the structures have graded d spacing across or through the structures.
These materials can be thought of as synthetic crystals or crystal analogues in which it is defined as crucial that the long range periodicity or repetition of a particular combination of layers be maintained. These structures are both structurally and chemically homogeneous in the x-y plane, and are periodic in the third (z) direction. These construction approaches particularly sputtering, can utilize a greater variety of materials than evaporation. The d spacing in a structure can be graded throughout the structure to provide some reflectivity for a range of X-ray wavelengths, but they do not achieve optimum control of higher order reflections and the deposition precision is not as good as desired. This results in interfaces and layer thicknesses which are not as precise as desired for certain applications. One desired goal in producing high efficiency X-ray reflectors is to produce a maximum contrast in electron density across the most precisely defined interface which produces the greatest number of orders of reflection. Further, the smoothness of the layer surface must be as precise as possible to minimize scattering caused by the surface variations.
X-ray dispersive structures and methods of making them are described in copending application, Improved X-ray Dispersive And Reflective Structures And Method Of Making The Structures, filed June 6, 1983, U.S. Ser. No. 501,659, John E. Keem et al., The layered structures described therein and the methods of making them are particularly applicable to making the focusing devices of the present invention. These structures and methods are discussed infra in detail beginning with reference to FIGS. 9 through 18.
Prior art and point source X-ray devices have been formed from cones which have an internal surface coated typically with W or Pt. The cones focus a very small amount of the X-ray intensity since the cones only collect and reflect X-rays which have a very small incidence angle (grazing incidence) which is below the angle of total internal reflection (.theta..sub.g).
A suggestion has been made to utilize lsm structures in imaging grazing angle incidence systems. A W:C layered structure on a paraboloid with a mean angle of 2.66.degree. was suggested for a telescope-monochromator for the Fe XXV line at 1.86 .ANG.. Underwood et al., Layered Synthetic Microstructures: Properties and Applications in X-Ray Astronomy, 184 SPIE 126 (1979).